The present invention relates to an apparatus for the continuous production of polyurethane block foam, in particular foams from fast-reacting, freely expanding reaction mixtures (having rise times of less than 1.5 minutes). The apparatus broadly comprises a continuous conveyor belt which together with adjustable lateral boundaries forms a width-adjustable expansion trough, a feeding table with a stationary mixing head disposed above it being arranged upstream of the conveyor belt. The feeding table has a distributing device which is disposed downstream of the mixture feeding point and comprises a deformable plate having, disposed below it, a height-adjustable support device, with one or more take-off stations being provided solely for bottom and side wall cover bands.
Continuous block foam production has currently reached a level which satisfies most of the requirements regarding quality and avoidance of waste through a rectangular block cross-section. Only a few special recipes require special measures which are still in need of improvement.
Problems still arise when processing fast-reacting reaction mixtures, e.g. soft, polyester-based foam mixtures, because here the rise time is only about one minute. The rise time is defined as the period from the start of combination of the reaction components up to the end of expansion (Plastics Manual, Vol.7, "Polyurethanes", 2nd Edition 1982, page 95, Carl Hanser Verlag Munich-Vienna). In comparison, the rise time for polyether foams is two to three minutes.
A short rise time requires a very narrow age spectrum of the applied reaction mixture over the expansion width as well as a uniform application height in order to achieve a rectangular block cross-section. The current standard expansion widths of up to 2.50 meters make it particularly difficult to meet said demands. Thus, for example, a uniform mixture application may be achieved by means of a mix head which oscillates over the expansion width but the age spectrum of the reaction mixture is then very disadvantageous. From European patent 21,159, it is known to calibrate the applied reaction mixture by means of a gap. The gap is delimited by the conveyor belt and a cylinder or beam disposed above the belt, with one of the two surfaces which form the gap being deformable from behind by means of adjusting screws so that the height of the gap is adjustable to differing degrees over the expansion width. To prevent the cylinder or beam from becoming soiled with reaction mixture, it is necessary to use not only a bottom, but also a top cover band. The reaction mixture accumulates in front of the cylinder or beam causing an undesirably broad age spectrum. In addition, at a high reaction rate the gases which arise cannot escape in time because of the top cover band and surface defects arise.
According to U.S. Pat. No. 3,123,856, when manufacturing rigid foam blocks, a rectangular block cross-section is achieved by forming, directly next to the lateral boundaries, recesses in the bottom band which are eliminated in the running direction of the conveyor belt. In this case also, the requirement for a reaction mixture age which is as uniform as possible over the expansion width is not met.
PCT WO 79/00186 discloses a width-adjustable block foaming installation having a fixed central part, whose side walls have inwardly displaceable bottom plates which catch under the central part so that the bottom band is applied over the entire expansion width. An oscillating mix head is used, resulting in a disadvantageous age spectrum. The bottom plates do not perform a mixture distributing function.
Finally, it is known (German Auslegeschrift 3,026,458) to dispose an elastic baffle plate below the outlet of a specially designed mixing device. The plate rests on a support with special contours for influencing the mixture distribution. An optimum mixture distribution is however impossible because the baffle plate is disposed above the feeding table and, more particularly, also above the bottom cover band. Since the baffle plate is fastened only in the central region, it is only roughly deformable. There is, in addition, a steep gradient which may lead to curtaining. Although the baffle plate is provided with an anti-adhesive coating, during production breaks the process of rinsing with solvents is unavoidable if the plate is to be kept clean.
The aim of the present invention is therefore to improve the continuous production of polyurethane block foam, in particular from a fast-reacting reaction mixture, i.e. in particular one having a rise time of less than 1.5 minutes, in such a way as to achieve as narrow as possible an age spectrum of the reaction mixture over the application width along with a uniform application height, in order to obtain a homogeneous foam quality with an at least approximately rectangular cross-section.